Many of the gigs I've done on bass where rigs have been provided, they have not been good rigs. They have been good makes but always had some problem, crackling signal, dodgy controls etc. I always use my own rig whenever possible.
I did a gig in London earlier this year and there was a big Ampeg rig with a 4x10 and 1x15 but when I plugged in it sounded dreadful and during soundcheck the signal kept breaking up at normal playing volume, which for me is not loud. So I spoke with the sound guy and he was fine with me using my own Mesa Walkabout head which I have learnt by experience to carry to all gigs where there is supposed to be a back-line provided. Saved my skin many times using as a head or even just taking it's DI out to the PA. At least I know my sound will be going out front even if not on stage.
As for allowing a band to use my rig, It's unlikely especially after reading this thread but maybe if it was a quiet band.
Like some have alluded to,I've spent a lot of time and money getting the rig I have and would not want the risk of someone blowing it and not thinking they should be paying for the repair or replacement.
Jazzyvee